Compass

This article is about the direction finding instrument used in navigation. For other uses, see Compass (disambiguation).

compass with a slit at cover and looking hole
A modern military compass, with included sight device for aligning

A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with magnetic north. Other methods may be used, including gyroscopes, magnetometers, and GPS receivers.

Compasses often show angles in degrees: north corresponds to 0°, and the angles increase clockwise, so east is 90°, south is 180°, and west is 270°. These numbers allow the compass to show azimuths or bearings which are commonly stated in degrees. If local variation between magnetic north and true north is known, then direction of magnetic north also gives direction of true north.

Among the Four Great Inventions, the magnetic compass was first invented as a device for divination as early as the Chinese Han dynasty (since c. 206 BC),[1][2] and later adopted for navigation by the Song dynasty Chinese during the 11th century.[3][4][5] The first usage of a compass recorded in Western Europe and the Islamic world occurred around 1190.[6][7]

The magnetic compass is the most familiar compass type. It functions as a pointer to "magnetic north", the local magnetic meridian, because the magnetized needle at its heart aligns itself with the horizontal component of the Earth's magnetic field. The magnetic field exerts a torque on the needle, pulling the North end or pole of the needle approximately toward the Earth's North magnetic pole, and pulling the other toward the Earth's South magnetic pole.[8] The needle is mounted on a low-friction pivot point, in better compasses a jewel bearing, so it can turn easily. When the compass is held level, the needle turns until, after a few seconds to allow oscillations to die out, it settles into its equilibrium orientation.

In navigation, directions on maps are usually expressed with reference to geographical or true north, the direction toward the Geographical North Pole, the rotation axis of the Earth. Depending on where the compass is located on the surface of the Earth the angle between true north and magnetic north, called magnetic declination can vary widely with geographic location. The local magnetic declination is given on most maps, to allow the map to be oriented with a compass parallel to true north. The locations of the Earth's magnetic poles slowly change with time, which is referred to as geomagnetic secular variation. The effect of this means a map with the latest declination information should be used.[9] Some magnetic compasses include means to manually compensate for the magnetic declination, so that the compass shows true directions.

  1. ^ Li Shu-hua, p. 176
  2. ^ Cite error: The named reference cambridge1 was invoked but never defined (see the help page).
  3. ^ Kreutz, p. 367
  4. ^ Cite error: The named reference needham volume 4 part 1 252 was invoked but never defined (see the help page).
  5. ^ Li Shu-hua, p. 182f.
  6. ^ Kreutz, p. 370
  7. ^ Schmidl, Petra G. (2014). "Compass". In Ibrahim Kalin (ed.). The Oxford Encyclopedia of Philosophy, Science, and Technology in Islam. Oxford University Press. pp. 144–146. ISBN 978-0-19-981257-8.
  8. ^ The magnetic lines of force in the Earth's field do not accurately follow great circles around the planet, passing exactly over the magnetic poles. Therefore the needle of a compass only approximately points to the magnetic poles.
  9. ^ "Declination Adjustment on a Compass". Rei.com. Retrieved 2015-06-06.